Modern digital integrated circuits can have internal logic disturbed by radiation in the form of alpha-particle or neutron strikes. A disruption is called a Single Event Upset (SEU). When SEUs occur in storage elements, such as memories, latches, flip-flops, and charge coupled devices, they are persistent. This persistence is particularly troublesome. The rate at which SEUs occur is called the Soft Error Rate (SER). Persistent SEUs in control logic, or state-machines, can lead a circuit into an unwanted state. This can lead to a system failure.
Radiation (e.g., alpha particle and neutron particle) flux changes with the physical environment and location. For example, cosmic radiation levels are correlated with altitude and thus can change significantly on airplane flights. In another example, free neutrons tend to follow the magnetic poles. Thus, neutron flux can vary significantly across the world. Thus, the SER also varies with the physical environment and location of the system or integrated circuit.